“Once social change begins,” says Cesar Chavez (Michael Peña), subject of Diego Luna’s perfunctory English-language directorial debut, “it can’t be reversed.” It took five years of strikes and boycotts and a 25-day fast, but Chavez and his United Farm Workers defeated the powerful California grape growers and compelled them to sign an equitable contract in 1970.

History, however, seems to have contradicted Chavez’s optimistic words. Today unions are virtually powerless, hostility to immigrants is more intense, and the disparity between rich and poor greater than ever. It no longer seems possible for mere courage, conviction, and determined organizing to achieve justice.

But I digress. Or maybe not, because Luna’s “Cesar Chavez,” with its yellowed, fuzzy “period” photography, its black-and-white archival inserts, its occasional, pseudo-verite use of handheld cameras, and its subdued performances and static narrative, plays more like an exercise in nostalgia than a dramatic re-creation of a triumphant fight for civil rights. The biopic follows Chavez’s story with a plodding, index card-like chronology, which is surprising since one of the writers, Timothy J. Sexton, co-wrote Alfonso Cuaron’s brilliant “Children of Men.” More surprising is the failure of Luna, an actor himself (best known for his role in Cuaron’s “Y Tu Mama Tambien”), to elicit more nuanced and passionate performances from his cast.

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Peña’s Chavez, for example, except for losing his moustache after moving out of the Los Angeles-based Community Service Organization office to the fields “where he started,” changes little throughout the film, bearing the same expression of weary determination. He explains in an opening monologue how his family lost their farm in the Depression and they were forced to work for the big growers and experienced first-hand the humiliation and outrage of being treated like chattel. But here his motives seem abstract and without zeal. When he demands that his union members not act macho and retaliate in kind against the violence of the anti-strike goons hired by the grape-growing nabob Bogdonovitch (John Malkovich), he seems less like Martin Luther King Jr. and more like a common scold. His hunger strike, meant to persuade his followers to embrace nonviolence, here seems confused and beside the point.

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As for the personal toll of his dedication and sacrifice, you’ve got to figure that with eight kids and a long-suffering wife (America Ferrera), his family life must have suffered. And indeed his oldest son resents being neglected and becomes estranged. But that barely registers as a footnote in the film, and the seven other kids seem lost in the shuffle.

So the years pass, with the strike and boycott gaining momentum with the nationwide support of everyday folks and charismatic politicians like New York Senator Robert Kennedy (Jack Holmes). Then RFK gets shot, Ronald Reagan (shown in news footage denouncing the boycott as immoral while munching on a handful of grapes) becomes governor of California, and Nixon is elected president. “We’re not going to survive Nixon,” says one of Chavez’s aides.

Somehow, they did. And if you can survive Nixon, maybe you can survive anything. But it will take a better movie than this to show how.